IMO, if your email works well everywhere, then the hosted web version link is just taking up valuable real estate at the very top. On the other hand, f you have a cool interactive email, then maybe you want it even more prominent than the ubiquitous top text link. As I mentioned in the #emailgeeks Slack, at my previous employer I had the brand logo also click through to the view-as-web hosted link, rather than to the home page. My reasoning was that they were getting a curated email with very specific content, and if they were just dumped into the homepage by an errant logo-click, they wouldn't be in the curated experience (and also, I wouldn't get clicks attributed). It was minorly controversial. But it shows another use to the hosted link than just "you have crappy software."
As Miah and Devon shared, MovableInk and LiveClicker have this functionality. I'd also put in a +1 for KickDynamic. Pricing models vary and it's based on the number of opens. You actually won't need to already have the user's location, as it will do a basic IP detection upon open and serve the corresponding forecast.
It probably depends on the amount of email and how much it overlaps with your regular 1:1 business email. If you're sending a high volume of email to a very broad audience of opt-ins, you may wind up tripping a block every once in awhile, and that might see your whole domain as a problem domain, which could spill over into other parts of your business. If you're small and just sending out to your customers, you are less likely to have a problem. You'll also want to make sure your SPF/DKIM/DMARC setups are tight regardless of domain.
Both are probably useful, but I would "officially" call option 1 "conversion rate" since it's the conversions out of your universe (people who got the email). It tells you overall from the subject line to the email content and call to action to the landing page experience how this particular campaign performed on a per-recipient basis. Conversions per click is narrower, but it can tell you if there's a problem with your landing page, for instance. It's taking out of the equation the effectiveness of the email itself in generating that click.
Jason and Kevin also mentioned this on the Email Design Podcast. I'm more concerned about the vertical space this takes up than the actual unsubscribe function. As you mentioned in your article, this functionality exists elsewhere and has yet to cause unsubageddon (sp?) and just pushes us email marketing to target more.
Are you also seeing the auto-scale not working on iOS10?
It could be that the clipping and open issue are related. (Even if not, you should address the clipping.) Determine how your email service provider records the opens: does it insert a 'beacon' image in the html and if so, where does it insert it? If the 'beacon' that records the open is at the bottom of your email, then you're losing recorded opens whenever it's clipped. Plus, clipping is showing that you've got too much in there.
If it's not a beacon issue, then the reasons that could easily account for the 8.57% difference are: (1) the Promotions tab (or other tab filtering) as you mentioned; or (2) Gmail's more aggressive spam filtering. 22.9% isn't bad though, I'd be grateful for that.